Page:The Journal of geology (IA journalofgeology11893univers).pdf/239
| Rate of erosion over land area of 600,000 square miles. | Time required for removal of 3,300 feet. | Rate of deposition in sea of 400,000 square miles, for 5,000 feet of strata. |
|---|---|---|
| 1 foot in 3,000 years, | 9,900,000 years | 1 foot in 1,980 years, or .006 inch per annum. |
| 1 foot in 1,000 years, | 3,300,000 years | 1 foot in 660 years, or .09 inch per annum. |
| 1 foot in 200 years, | 660,000 years | 1 foot in 132 years, or .18 inch per annum. |
The rate of one foot in 200 years is assumed as the most probable and 660,000 years as the time required for the removal and deposition of the 5,000 feet of post-Cambrian mechanical sediments.
There is one factor that may need to be taken into consideration in estimating the time duration of the deposition of the mechanical sediments of the Cambrian and pre-Cambrian of the northern portion of the Cordilleran sea that would materially lengthen the period. Dr. George M. Dawson describes the Nisconlith series, especially in the Selkirk range of British Columbia, as composed of "blackish argillite-schists and phylites, generally calcareous, with some beds of limestone and quartzite, 15,000 feet."[1] It is correlated with the Bow River series, which contains, in the upper portion, the lower Cambrian fauna. The presence of these calcareous beds indicates a slower rate of deposition than we have estimated for the lower portion of the Cambrian series over the greater part of the Cordilleran sea; but as yet the correlation with the sediments of the Cordilleran sea is not sufficiently well established to warrant our allowing a greater time period to the Cambrian on this account.
Estimates from Chemical Sedimentation.—We have estimated that the Paleozoic sediments of the Cordilleran sea contain 2,007,244,800 million tons (900 million mile-feet) of carbonate of lime, which was derived by organic or chemical agencies from the sea water to which it was contributed by the land. If oceanic circulation could be excluded from the problem we might pro-
- ↑ Bull. Geol. Soc. Amer., Vol. II., 1891, p. 168.